The present invention relates to laser devices comprising a nitride semiconductor, and more particularly to nitride semiconductor laser devices adapted for self-pulsation.
Semiconductors laser devices have found wide application to optical communication, optical recording and various other fields and are prevalent used especially in the field of optical recording chiefly in optical disk systems. In the case where semiconductor laser devices are used as light sources for optical disk systems, the diameter of the spot of light focused on the optical disk by an objective lens is directly proportional to the lasing wavelength of the semiconductor laser device, so that the shorter the lasing wavelength, the smaller the spot diameter on the optical disk is and the greater the recording capacity of the disk is. While AlGaInP semiconductor laser devices heretofore in actual use are about 650 nm in lasing wavelength, nitride semiconductor laser devices are as short as about 400 nm in lasing wavelength. Accordingly progress has been made in developing nitride semiconductor lasers to achieve increased recording capacities in optical disk systems.
Already known as semiconductor laser devices having a double heterojunction structure are the ridge stripe semiconductor laser device shown in FIG. 11, and the self-aligned semiconductor laser device shown in FIG. 12.
With reference to FIG. 11, the ridge stripe semiconductor laser device comprises an n-type cladding layer 3, active layer 4, p-type cladding layer 561, n-type current blocking layer 611 and p-type contact layer 7 which are superposed on a substrate 10, for example, of sapphire. The p-type cladding layer 561 has an upwardly projecting ridge stripe portion 50.
As shown in FIG. 12, the self-aligned semiconductor laser device comprises an n-type cladding layer 3, active layer 4, p-type first cladding layer 57, n-type current blocking layer 62, p-type second cladding layer 58 and p-type contact layer 7 which are superposed on a substrate 10, for example, of sapphire. The p-type second cladding layer 58 has a downwardly projecting ridge stripe portion 60.
The ridge stripe semiconductor laser device is fabricated by the process shown in FIG. 13, (a) to (d). First as seen in FIG. 13, (a), an n-type contact layer 2, n-type cladding layer 3, active layer 4 and p-type cladding layer 59 are formed as superposed on a substrate 10, and a mask 15 is then formed on the surface of the p-type cladding layer 59. Next as shown in FIG. 13, (b), the p-type cladding layer 59 is dry-etched with the mask 15 provided thereon to form a ridge stripe portion 50. Subsequently as seen in FIG. 13, (c), an n-type current blocking layer 611 is selectively grown utilizing the mask 15 remaining on the ridge stripe portion 50, the mask 15 is thereafter removed, and a p-type contact layer 7 is grown as shown in FIG. 13, (d).
In fabricating AlGaAs infrared semiconductor laser devices or AlGaInP red semiconductor laser devices, the ridge stripe portion 50 can be formed with high accuracy by inserting in advance into the p-type cladding layer an etching stop layer which is more resistant to a specified etchant than the p-type cladding layer. However, since no suitable etchant has been found for use in fabricating nitride semiconductor laser devices, it is difficult to form the ridge stripe portion by such chemical etching. It is therefore conventional practice to form the ridge stripe portion 50 by dry-etching the p-type cladding layer 59 as seen in FIG. 13, (a) and (b).
The self-aligned semiconductor laser device is fabricated by the process shown in FIG. 14, (a) to (c). First as seen in FIG. 14, (a), an n-type contact layer 2, n-type cladding layer 3, active layer 4, p-type first cladding layer 57 and n-type current blocking layer 63 are formed as superposed on a substrate 10, and masks 16, 16 are formed on the surface of the n-type current blocking layer 63. Next as shown in FIG. 14, (b), the n-type current blocking layer 63 is dry-etched through the masks 16, 16 to locally remove the current blocking layer 63 to a depth where the p-type first cladding layer 57 becomes exposed. Subsequently as shown in FIG. 14, (c), the masks 16, 16 are removed to grow a p-type second cladding layer 58 and p-type contact layer 7.
However, the fabrication of the ridge stripe semiconductor laser device requires three crystal growth steps. It is also required to control the thickness t of the p-type cladding layer 561 beneath the n-type current blocking layer 611 by dry etching, whereas difficulty is encountered in controlling this thickness. If the thickness of the cladding layer 561 is smaller than the proper value, it is difficult to effect the self-pulsation to be described below, while if the thickness of the cladding layer 561 is greater than is proper, the n-type current blocking layer 611 to be grown subsequently will have a smaller thickness, failing to effectively block the current.
On the other hand, two crystal growth steps suffice to fabricate the self-aligned semiconductor laser device. Furthermore, the thickness of the p-type cladding layer 57 beneath the n-type current blocking layer 62 can be controlled by crystal growth. With this type of laser device, nevertheless, dry etching for locally removing the current blocking layer is likely to cause damage to the light-emitting portion and current injection region, and such damage is likely to produce an adverse effect on the emission of light.
It is required that the optical disk player comprising a semiconductor laser device as its optical component be low in noise against optical feedback from the optical disk. Accordingly it is practice to obtain multimode operation by superposing high-frequency waves of about several hundreds of megahertz to about 1 GHz on the drive signal for a semiconductor laser device of the single-mode pulsation type, or to cause semiconductor laser devices to undergo self-pulsation (see, for example, xe2x80x9cTheoretical Analysis of Self-Pulsation Phenomena in Semiconductor Lasers,xe2x80x9d Technical Report of Society of IEICE, OQE92-16).
The method of obtaining a multimode by the superposition of high-frequency waves requires an external circuit, so that the method of effecting self-pulsation is desirable from the viewpoint of compacting the device and cost reductions. Self-pulsation occurs by forming a light absorbing region termed a saturable absorbing region in the active layer around the region thereof where current is injected (active region).
Accordingly used in fabricating ridge stripe or self-aligned semiconductor laser devices is a method of giving a spot width greater than the width of the current injection region to effect self-pulsation, by optimizing device structure parameters such as the stripe width W, the thickness t of the p-type cladding layer 561 beneath the n-type current blocking layer 611 and the thickness d of the active layer 4.
Also used is a method of causing side portions of the current injection region within the active layer to act as a saturable absorbing region by giving the active layer an increased thickness.
With the ridge stripe semiconductor laser device shown in FIG. 11, however, the ridge stripe portion 50 flares downward in cross section, so that current is injected as spread out along this configuration into the active layer 4. As a result, a current injection region is provided which spreads out beyond the width W of the stripe portion 50 at the position of its lower end, and the width of the current injection region becomes approximately equal to the spot width.
Further with the self-aligned semiconductor laser device of FIG. 12 wherein the ridge stripe portion 60 is tapered downward in cross section, current is injected as confined in this configuration into the active layer 4. Consequently, a current injection region is provided which is narrower than the width W of the stripe portion 60 at the position of its lower end. Since the spot width is also diminished, the width of the current injection region becomes approximately equal to the spot width.
Thus in the case of the conventional semiconductor laser devices, the width of the current injection region becomes approximately equal to the spot width whether the device is of the ridge stripe type or self-aligned type, so that the optimization of device structure parameters such as the stripe width W of the ridge stripe portion, the thickness t of the cladding layer and the thickness d of the active layer encounters the problem that the freedom of selecting the parameters is small.
In order to fabricate self-pulsation semiconductor laser devices in a high yield, there is a need to give accurate parameters to each device structure as finished, whereas the accuracy of the stripe width W is dependent on the dimensional accuracy of the mask, that of the cladding layer thickness t on the controllability of etching and that of the active layer thickness d on the controllability of the thickness to be obtained by crystal growth. Because difficulty is encountered especially in controlling the amount of layer to be removed by dry etching for forming the ridge stripe portion, it is impossible to obtain the cladding layer thickness t with high accuracy, hence the problem of a low yield.
Further increasing the thickness of the active layer gives rise to the problem of increased threshold current or a greater astigmatic distance.
Accordingly, an object of the present invention is to provide a nitride semiconductor laser device of the self-pulsation type which has a short lasing wavelength and is not increased in threshold current and astigmatic distance and which can be fabricated in a higher yield than conventionally.
The present invention provides a nitride semiconductor laser device comprising a first cladding layer 3 formed on a substrate and made from a first conductive-type nitride semiconductor, an active layer 4 formed on the first cladding layer 3, and a second cladding layer 5 formed on the active layer 4 and made from a second conductive-type nitride semiconductor. The second cladding layer 5 comprises a flat portion 5a formed over the active layer 4 and a dual stripe portion 53 projecting upward from the midportion of the flat portion 5a, and a current blocking layer 6 of a first conductive-type nitride semiconductor is formed at each of opposite sides of the dual stripe portion 53.
The dual stripe portion 53 of the second cladding layer 5 comprises a lower stripe portion 52 formed on the flat portion 5a and an upper stripe portion 51 formed on the lower stripe portion 52. The dual stripe portion 53 varies in cross sectional area toward the flat portion 5a. The rate of variation in the cross sectional area of the portion 53 increases toward a positive direction from the position of the boundary between the upper stripe portion 51 and the lower stripe portion 52.
The term xe2x80x9cfirst conductive-typexe2x80x9d means one of p-type and n-type, and the term xe2x80x9csecond conductive-typexe2x80x9d means the other of these types.
With the nitride semiconductor laser device of the invention described, the second cladding layer 5 has a dual stripe portion 53 comprising an upper stripe portion 51 and a lower stripe portion 52, and the rate of variation in the cross sectional area of the dual stripe portion 53 increases in the positive direction from the position of the boundary between the upper and lower stripe portions 51, 52, so that the upper stripe portion 51 is smaller than the lower stripe portion 52 in cross sectional area at least in the vicinity of the boundary position.
Since it is difficult to give an increased carrier concentration to the p-type semiconductor layer especially in nitride semiconductor laser devices, the current confined in the upper stripe portion 51 encounters difficulty in spreading out laterally.
Accordingly, the current flowing through the dual stripe portion 53 of the second cladding layer 5 toward the active layer 4 flows into the active layer 4 while being restrained by the upper stripe layer 51 from spreading out laterally. Further because the lower stripe portion 52 has a greater width than the upper stripe portion 51, the width of the spot to be formed in the active layer 4 becomes greater than the width of the current injection region.
Consequently, a saturable absorbing region is formed around the current injection region, giving rise to self-pulsation.
The rate of variation in the cross sectional area of the dual stripe portion 53 increases toward the positive direction from the position of the boundary between the upper stripe portion 51 and the lower stripe portion 52, for example, in an embodiment wherein the upper stripe portion 51 is made approximately constant or decreased in cross sectional area toward the flat portion 5a, and the dual stripe portion 53 is markedly increased in cross sectional area at the position of the boundary between the upper stripe portion 51 and the lower stripe portion 52.
The present invention provides another nitride semiconductor laser device wherein a second cladding layer 5 has a dual stripe portion 53 comprising a lower stripe portion 52 formed on the flat portion 5a and an upper stripe portion 51 formed on the lower stripe portion 52, the width of the upper stripe portion 51 in a direction orthogonal to the longitudinal direction thereof being smaller than the width of the lower stripe portion 52 in the same orthogonal direction.
For example, the upper stripe portion 51 has a minimum width W1 at the position of the boundary between the upper and lower stripe portions 51, 52, and the lower stripe portion 52 has at the position of its lower end a width W2 greater than the minimum width W1 of the upper stripe portion 51.
Alternatively, the lower stripe portion 52 projects outward beyond each of opposite side faces of the upper stripe portion 51 at and below the position of the boundary between the upper and lower stripe portions 51, 52, and the projection has an upper face substantially parallel to an upper surface of the active layer 4, the lower stripe portion 52 having a greater width than the upper stripe portion 51 at the position of the boundary between the upper and lower stripe portions 51, 52.
When the dual stripe portion 53 is thus constructed, the width W1 of the upper stripe portion 51 is smaller than the width W2 of the lower stripe portion 52, so that the current flowing through the dual stripe portion 53 toward the active layer 4 is restrained by the width W1 of the upper stripe portion 51 and flows into the active layer 4 while being restrained from spreading out laterally. Accordingly, the current injection region of the active layer 4 has a width in conformity with the width W1 of the upper stripe portion 51.
On the other hand, the lower stripe portion 52, which is enlarged as compared with the upper stripe portion 51, laterally increases the spot width, with the result that the spot width of the active layer 4 is made to conform to the width W2 of the lower stripe portion 52 and made greater than the width of the current injection region.
Consequently, a saturable absorbing region is formed around the current injection region to give rise to self-pulsation.
Each current blocking layer 6 can be formed from a material greater in band gap energy and smaller in refractive index than the second cladding layer 5. Alternatively, the current blocking layer 6 can be formed from a material smaller than the active layer 4 in band gap energy and guides laser light by absorbing light generated in the active layer 4 in this case.
According to an embodiment, the dual stripe portion 53 of the second cladding layer 5 is so formed that the longitudinal direction of the configuration of the stripe thereof is the direction [1 1 {overscore (2)} 0] of nitride semiconductor crystals, and the upper stripe portion 51 decreases in cross sectional area toward the flat portion 5a. 
In this case, a sapphire substrate or GaP substrate is usable as the substrate.
According to another embodiment, the dual stripe portion 53 of the second cladding layer 5 is so formed that the longitudinal direction of the configuration of the stripe thereof is the direction [1 {overscore (1)} 0 0] of nitride semiconductor crystals, and the upper stripe portion 51 is approximately constant in cross sectional area toward the flat portion 5a. 
Usable as the substrate in this case is a GaN substrate, Si substrate, 6Hxe2x80x94SiC substrate, 4Hxe2x80x94SiC substrate, MgO substrate or MgAl2O4 substrate.
With the nitride semiconductor laser device according to the present invention, the current injection region width can be defined by the upper stripe portion, and the spot width by the lower stripe portion, so that the device of the invention can be caused to generate self-pulsation more easily than conventional semiconductor laser devices. The construction of the invention therefore gives a higher degree of freedom in optimizing the device structure parameters and affords greater allowable ranges of dimensional accuracies to the parameters than in the case of conventional semiconductor laser devices, making it possible to fabricate self-pulsation semiconductor laser devices in a high yield. Since there is no need to give an increased thickness to the active layer for self-pulsation, self-pulsation can be realized without increasing the threshold current or astigmatic distance.